Gall-Tinsley Coordinate Type Potentiometer - Construction & Working

     A coordinate type potentiometer is a combination of two potentiometers. One of the potentiometers carries a current in-phase with the supply voltage and it is called an 'In-phase Potentiometer'. The other potentiometer carries the current in quadrature with supply voltage and it is called a 'Quadrature Potentiometer'.

Gall-Tinsley Coordinate Type Potentiometer :

     The connection diagram of this potentiometer is shown in the below figure. T1 and T2 are the two step-down transformers fed from a single-phase supply. The supply to T2 is obtained through the series combination of variable capacitor Cs and variable resistor Rs for splitting the phase. The exact quadrature in phase is obtained by adjusting Rs and Cs. Here, ab and cd are sliding contacts of In phase and Quadrature potentiometer respectively, and rheostats R1 and R2 are used for current adjustments.

Co-ordinate Type Potentiometer

     VG is a vibrational galvanometer tuned to the supply frequency. The ammeter A (reflecting electro-dynamometer type) ensures current in both In-phase and Quadrature Potentiometers slide wire at standard value. Similarly, the reversing switches RS 1 and RS 2 of two potentiometers are used to reverse the direction of the unknown emf across its slide wires. S2 is a selector switch for placing unknown voltages to be measured in the circuit.

     The In-phase potentiometer measures the component of unknown voltage which is in phase with its slide wire current. Let its value be V1 and the component which is in phase with the Quadrature potentiometer current is measured on it and it is the quadrature component of unknown voltage. Let its value be V2.

     Then the magnitude of the unknown voltage and phase angle with respect to supply voltage is given by,

Co-ordinate Type Potentiometer

Standardization of Potentiometer :

     The dc standardization of the in-phase potentiometer is done by connecting the battery B through the switch S1 and changing the multiple circuit switch S2 to position 1-1. The vibrational galvanometer is replaced by a galvanometer for this purpose. The electro-dynamometer ammeter is tuned to zero position on direct current and this setting is left untouched.

     The switches S1 and S2 are again brought back to the initial position. The alternating current is adjusted in the in-phase potentiometer by rheostat R1 to give zero deflection of the milli-ammeter. The magnitude and phase of the quadrature potentiometer's current are adjusted by the mutual inductor M.

     The switch S2 is brought to position 3-3. The dial settings of the in-phase potentiometer are done to read a value of M i (∵ i is the primary current, emf induced in the secondary winding = 2π f Mi). Where i is the standard alternating current in the in-phase potentiometer.

     The magnitude and phase of the current in the quadrature potentiometer are adjusted through rheostat R2 and variable resistance Rs of the phase splitting device to obtain the exact balance which is indicated by the vibration galvanometer.

     The switch S2 is again brought to position 2-2. In this position, two slide circuits and a vibration galvanometer are in series with the unknown voltage. Now, the potentiometer is ready to measure the two components of unknown voltage. The balance is obtained by adjusting the settings of sliding contact a and c together with the reversing switches RS 1 and RS 2 if necessary.

Errors in Coordinate Type Potentiometer :

The errors developed in a coordinate potentiometer are due to the following reasons,
  • As we know that, different parts of the instrument will have mutual inductance. This will affect the nominal value of mutual inductance, which in turn produces errors.
  • Inaccurate measurement of frequency will produce an error.
  • Due to the presence of harmonics in the supply, errors are introduced.
  • Inter-capacitance, earth capacitance, and mutual inductance of the slide wire coils will affect the potential gradient, which may produce an error.
  • Even a very minute difference in ac and dc reading of the reflecting dynamometer instrument may introduce errors.

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